In hydrate-based or clathrate-based desalination (referred to herein simply as “hydrate-based desalination”), fresh water is “extracted” from saline or brackish water by forming hydrate in the saline or brackish water, gathering the hydrate, and allowing it to dissociate to release the fresh water and the hydrate-forming substance (which may be introduced to the water to be treated as a liquid or, more preferably, as a gas). Although hydrate-based desalination has been known conceptually since as early as the late 1950's, it is believed that, to date, it has not been possible to conduct hydrate-based desalination on a commercially viable basis, i.e., in a manner that produces water of potable quality over sustained periods of time at volumetric rates sufficient to justify the construction of a commercial-scale installation.
As far as I know, according to previous methods, the hydrate-forming substance (e.g., gas) has been simply injected into the water to be treated in relatively uncontrolled fashion, e.g., as bubbles. When the hydrate-forming substance is so injected into the water to be treated under appropriate hydrate-forming conditions, hydrate quickly nucleates on the bubbles and grows to form a thin shell of hydrate around the bubble. Growth ceases, however, when the hydrate shell reaches a thickness on the order of five or ten to twenty microns, thus effectively separating the bubble of hydrate-forming substance from the surrounding water to be treated. At that point, further hydrate growth, if any, becomes dependent on the much slower, inefficient process of gas or water diffusion through the hydrate shell.
The hydrate shells do tend to fracture, however. In particular, they can fracture due to colliding with each other in the turbulent motion of the water to be treated as hydrate-forming substance is injected (particularly where it is injected in liquid phase and the hydrate-forming substance spontaneously and violently changes to its gas phase). Additionally, the bubbles may fracture as positively buoyant hydrate-encrusted bubbles of hydrate-forming substance (particularly gas) rise through the water column into regions of reduced ambient pressure, where the hydrate-forming substance can expand and fracture the hydrate shells. (In the case of shells formed from negatively buoyant hydrate, the shell-encrusted bubbles may sink to regions of increased ambient pressure and fracture due to that increased pressure.) As a result, what is obtained is essentially a slurry of thin fragments of hydrate shells, and that result is undesirable for a number of reasons.
First, where the hydrate forms a slurry of shell fragments, the slurry traps or carries a large amount of residual brine of elevated salinity in the interstitial spaces between the shell fragments. That residual brine is difficult to expel completely using a washing process and mixes with the fresh water released by the hydrate as it dissociates, thus raising the salinity of the product water to levels that may prevent it from being potable.
Second, I have recognized that in order to maximize the amount of fresh water actually recovered from a given mass of hydrate, as much of the hydrate as possible should dissociate near the point of fresh water collection. Because hydrate dissociation is a surface event or phenomenon—in other words, hydrate dissociates or decomposes from its surface inwardly, in contrast to the whole mass of hydrate breaking apart when it moves into a region where it is no longer stable—I have recognized that it is important to minimize the ratio of hydrate surface area to hydrate mass in order to obtain as much fresh water as possible from a given mass of hydrate. When the product hydrate exists largely in the form of a slurry of shell fragments, the hydrate surface area is quite high relative to the hydrate mass. As a result, the hydrate begins to dissociate long before it can be removed to an ideal dissociation location, e.g., typically in a region where much contaminating residual brine is present. Additionally, the hydrate tends to dissociate at a rate that is too high for the process to be well controlled or to be able to recover as much fresh water as possible.
Additionally, some of my earlier work relating to using positively buoyant hydrate for the desalination process has relied on or taught using the natural buoyancy of the hydrate to collect it by simply allowing the hydrate to float upward into a region where it is no longer stable, at which point the hydrate dissociates to release fresh water and the hydrate-forming substance. When the hydrate is in the form of small shell fragments or shards, however, the amount of fluid drag on the hydrate may be so great that buoyant upward movement of hydrate is retarded or even prevented if relative flow and hydrate formation rates are not controlled properly. In such a situation, the slurry of hydrate shells and residual brines would separate from each other only over long periods of time, which would make hydrate-based desalination impractical, or by using mechanical means such as centrifugal dewatering, which would make the process complex and more expensive.
I believe that these factors have inhibited the use of hydrate-based desalination on a commercial basis, i.e., on a continuous basis and/or on a large enough scale to provide fresh water for a whole community.